
<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <title>Title</title>
</head>
<body>
<form action="cal.php" method="get">
<input type='text' name="a1"/>
<select name="a2">
    <option>+</option>
    <option>-</option>
    <option>*</option>
    <option>/</option>
</select>
<input type='text' name="a3">
<input type='submit'>

</form>
</body>
</html>